1. Field of the Invention
The present invention relates to a data read/write device for a holographic storage medium, and more specifically, to a read/write device and method thereof, which enables a low-cost compact optical pick-up system for a holographic storage medium by use of focusing lens unit having a multi-focal lens.
2. Description of the Related Art
With the widespread use of computers, development of peripherals and components that enhance computer operation has increased. Among these efforts, there is a supplemental storage medium which keeps program or information therein even in the event of computer power off. There are a variety of supplemental storage media. A magnetic storage medium and an optical disk storage medium are the representative examples of such supplemental storage media.
The magnetic storage medium includes floppy disk and hard disk, which stores data therein by use of magnetic force. The hard disk has a large memory capacity, and is fast in inputting and outputting information, and therefore, is used as a main supplemental memory device for a personal computer. The floppy disk is convenient to carry and therefore, has been widely used as a supplemental memory device of a personal computer in the past. However, several tens of floppy disks at maximum capacity are frequently required to store multimedia data due to the floppy disk's limited memory capacity. Therefore, the floppy disk is not suitable for the current memory storage trend.
In order to overcome the limited memory capacity of the floppy disk, an optical disk device, or a laser disk device, has been developed. The optical disk has a relatively large memory capacity, and enables a fast search. Therefore, the optical disks are widely used these days. There are representatively a CD-ROM and a digital versatile disk (DVD) as for the optical disks.
However, the rapid advancement in the field of computer and information communications technologies requires storage means of more vast memory capacity. The data which reaches up to several billions of bytes, easily exceeds the storage capacity of a simple CD, and therefore, a new storage medium is now in demand. Recently, a holographic information storage that utilizes the photorefractive effect, which refers to a local change in the index of refraction of the material due to incident light, has been suggested.
The holographic information storage stores information in a photosensitive non-organic crystal or a polymer material in the form of optical interference patterns. The optical interference pattern is formed by use of two laser beams which interfere with each other. In other words, a reference beam containing no data and an object beam containing a data therein interfere with each other, forming an interference pattern. As the interference pattern causes chemical or physical changes in the photosensitive storage medium, the data is stored. A storage medium storing an image of object in the interference pattern or the interference pattern itself is called a hologram, and a holography relates to an art of using the hologram.
Meanwhile, a storage medium using holography can store information in a three-dimensional space of a photorefractive crystal. Because multiple holograms can be multiplexed in the same region of the storage medium in accordance with angles, phases and wavelengths of the reference beam, data of high density can be reproduced with speed and storage capacity is also improved.
The hologram information storage can be divided into a page-based storage and a bit-based storage in accordance with the specific method of storage as employed. The page-based storage forms bit data into a two-dimensional page, and records data by use of a spatial light modulator (SLM). The bit-based storage modulates the light intensity of the object beam according to the binary data or to the coded data. In the page-based storage, a vast amount of data can be recorded or reproduced, and therefore, high speed data input and output can be obtained. However, the price of devices such as a SLM for data input, or a data detector for the reproduced page, is too high to be commercialized at the consumer level. Furthermore, it is difficult to ensure the satisfactory compatibility with the existing optical disk drive.
Meanwhile, the bit-based storage system attracts attention of the industry. In WORM (write once and read many) system, in particular, the user can directly write the data to reproduce, and therefore, it is unnecessary to adopt a writing method as fast as the page storage. As a result, it is possible to realize an economic bit-based storage system.
FIG. 1 shows the structure of a conventional data write/read device which writes data to holographic WORM according to bit-based storage method.
Referring to FIG. 1, a data write/read device includes a laser beam emitting device 40, a phase conjugator 20, a disk rotation axis 30, and an optical signal detecting device 50. A holographic storage medium 10 is made to the disk type, and aligned on the phase conjugator 20.
The laser beam emitting device 40 includes a light source 41, a beam splitter 43, and an objective lens 45. When a laser is emitted from the light source 41, the laser beam changes its path as it passes through the beam splitter 43, and coverges on a predetermined area of the holographic storage medium 10.
As the laser beam is emitted to the holographic storage medium 10, a part of the laser beam is transmitted through the holographic storage medium 10 and then reflected by the phase conjugator 20 provided at the lower side and therefore, enters into the holographic storage medium 10. Accordingly, the reflected laser beam from the phase conjugator 20 interferes with the laser beam emitted from the laser beam emitting device 40, generating an interference pattern. The interference pattern is recorded in the storage medium 10 as hologram data.
FIG. 2 shows an enlargement of an encircled area ‘a’ of the holographic storage medium 10 of FIG. 1 to which a laser beam is converged. Referring to FIG. 2, a laser beam is converged to the holographic storage medium 10 and forms a waist at a predetermined depth of the holographic storage medium 10. The laser beam is reflected in the direction of entering. As a result, interference of laser beams occurs, and holographic data is recorded in the holographic storage medium 10.
The holographic storage medium 10 is formed to a disk type, which rotates about the disk rotation axis 30, varying the surface where the laser beam enters and therefore, recording and reproducing data.
Meanwhile, in order to reproduce data, a laser beam is emitted from the laser beam emitting device 40 to the holographic storage medium 10 at the identical wavelength as that of the data recording. The object beam is reproduced as if it comes from the phase conjugator 20, the data is to be analyzed by detecting the object beam through the photo detector 50.
In the conventional art, however, the phase conjugator 20 is made from a single crystal structure, and therefore, it is quite difficult and expensive to make the phase conjugator 20 as large as the holographic storage medium 10. Accordingly, a lot of doubts still remain for the commercialization. Furthermore, because the depth of the waist of the laser beam must be multiplexed for high recording density, multi-layered optical disk technology is required. However, multi-layered recording becomes difficult to achieve if recording requires several tens of layers, instead of several layers.